One process for obtaining a powdery detergent includes a process comprising the step of supporting a liquid surfactant in particles for supporting a surfactant. In this process, a high supporting ability of the liquid surfactant is demanded for the particles for supporting a surfactant. In other words, there are two factors for the supporting ability demanded for the particles for supporting a surfactant: A large amount of a liquid surfactant can be retained (supporting capacity); and the liquid surfactant once absorbed can be strongly retained in the inner portion of the particle without being bleeded out (supporting strength). The supporting capacity is important from the viewpoint of formulating a surfactant in an amount necessary for detergency performance, and the supporting strength is also important from the viewpoints of preventing lowering the flowability of powdery detergent, caking, and migration of the liquid surfactant to a container or its surface by suppressing the bleed-out of the liquid surfactant.
Further, from the viewpoint of productivity, a property of quickly absorbing the liquid surfactant (supporting rate) is also demanded for the particles for supporting a surfactant.
As to the structure demanded for the particles for supporting a surfactant having a high supporting ability, it is desired to have a structure so that the supporting capacity is increased by having a sufficient microporous capacity in the inner portion of the particle, and that the supporting strength is high by having fine micropore diameter. Such a structure is obtained by constructing the particles for supporting a surfactant with fine particles such that the particles are in contact with each other, with maintaining a sufficient air gap therebetween. As a supplying source for the fine particles, a water-soluble salt in a detergent composition can be utilized. For instance, a representative water-soluble salt usable for a detergent composition includes sodium carbonate. Sodium carbonate forms sodium carbonate monohydrate or burkeite, which is a compound salt with sodium sulfate, in a slurry, these compounds can form fine acicular crystals to serve as a base material for forming an effective supporting site in the inner portion of the particle for supporting a surfactant.
As a technique for actualizing such formation, Japanese Patent Laid-Open No. Sho 62-112697 discloses a process of obtaining a dry powder having a high adsorption capacity (particles for supporting a surfactant), comprising adding and mixing a crystal growth-controlling agent, which is an organic substance having at least 3 carboxyl groups in the molecule, in an effective amount, with a slurry, prior to mixing the slurry with sodium carbonate, thereby forming sodium carbonate monohydrate and/or burkeite, of which crystal growth is controlled, in the slurry; and thereafter spray-drying the mixture slurry.
However, the supporting ability of the particles for supporting a surfactant obtained by this process has not been sufficient. The causes therefor include the amount of the fine burkeite dispersed being insufficient in the slurry before spray-drying; and the amount of the fine acicular crystals of burkeite being insufficient also in the particle obtained by spray-drying. The fine burkeite crystals are a base material effective for improving the supporting ability. However, in this technique, since dissolved sodium sulfate forms burkeite on the surface or near the surface of granular sodium carbonate added afterwards, a majority exists as an aggregate which is hard and has a large particle size. Therefore, the amount of the burkeite in a fine acicular crystal state formed in the slurry is small, and the burkeite which could have been inherently formed into fine acicular crystals takes an aggregated state having a large particle size in the particle even after spray-drying. Therefore, the resulting particles have large microporous capacity and micropore diameter, so that a sufficient supporting ability cannot be exhibited.
Also, a polyacrylate (polymer), which is a polymer especially effective as a crystal growth-controlling agent, may form a coating film on the particle surface. Therefore, when the polymer is formulated as a detergent composition in an effective amount or more, there may be some cases where the resulting particle does not exhibit a sufficient supporting ability. In this publication the maximum supporting capacity is exhibited when the amount of the polymer in the particle is as small as about 1 to about 2% by weight, so that a certain limitation must have been added to the formulation amount of the water-soluble polymer.
The water-soluble polymer is a base material having a film-forming characteristic by drying. When the water-soluble polymer is formulated in the slurry, a coating film containing a water-soluble polymer on a particle surface after drying is formed, thereby lowering the degree of porocity. In this case, the supporting rate tends to be lowered, so that a certain period of time has been required for sufficiently supporting a liquid surfactant in the particles for supporting a surfactant. In order to efficiently prepare detergent particles by the process of supporting the liquid surfactant in the particles for supporting a surfactant, it has been desired to further increase the supporting rate for the liquid surfactant composition in the particles for supporting a surfactant.